In a color thermal printing process, the finished print is made by successively transferring yellow, magenta, and cyan dyes from respective dye donors onto a dye receiver. Because the dyes are applied as color separations from the dye donors one at a time, it is desirable to align or register each color separation with the previous color separation so that the finished print is perfectly aligned and crisp. Earlier thermal printers had large drums to which a sheet of receiver media was attached. The drum was then rotated as each dye color separation was applied. The drum always rotated in the same path and it was fairly easy to achieve acceptable color registration. However, a drum large enough to accommodate a regular letter size sheet of paper caused the printer to be a bulky machine.
More modern printers have smaller platen-styled printing drums, on the order of an inch or so in diameter. The reduced size of the printing drum creates problems with color registration. There are several printer transport mechanisms that include capstans and pinch rollers for controlling the receiver during printing, as disclosed in U.S. Pat. Nos. 4,881,831; 4,755,833; 4,532,525; and 4,505,603. These small platen printer patents disclose transport systems for transporting the dye receiver material past the print head. While acceptable image reproduction quality is achievable, each of these prior art patents uses a continuous roll, and the technology necessary to print high resolution, high density images cannot be achieved in a single sheet system by the prior art devices.
In prior printers, dye transmission errors are created when the tension in the section of the dye receiver between the print head and the receiving media control system varies from slack to taut, repeatedly. Such tension variation will cause receiver speed changes at the print head; resulting in undesirable light and dark printed bands at the same frequency.
Another problem with the prior art small print drum printers is curling of the completed print. Curling is thought to be caused by the high temperatures necessary to achieve darken image printing. It will be appreciated that it is highly desirable to have a thermal printer which produces high quality prints without light or dark bands and without curling.
Since each of these prior art patents cited above uses a continuous roll, there is no possibility of having the wrong length of receiver in the machine. But single sheet fed thermal printers are also known, and it would be highly desirable to have a feature in such a sheet fed thermal printer that would prevent the operator from using the wrong length of dye receiver.
Another problem in conventional thermal printing is improper receiver position perpendicular to the path of motion, known as skew. Skew results in the image being mispositioned in a side to side fashion on the dye receiver. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer wherein good side to side positioning of the image on the media is assured.